Acetogenesis

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Acetogenesis is a special form of energy metabolism , namely anaerobic respiration , which is based on the reductive acetyl-CoA pathway (Wood-Ljungdahl pathway). Carbon dioxide (CO 2 ) acts as an electron acceptor and thus takes on the function of an oxidizing agent that O 2 has in aerobic respiration . The end product of this “CO 2 breathing” is (mostly) only acetic acid excreted. In contrast to other microbial anaerobic processes in which acetic acid is not formed as the main product, this process is also known as homoacetogenesis .

Fig. 1. Ecophysiology of acetogenesis (solid arrows)
A fermenting anaerobe ( F ), which obtains ATP exclusively through substrate chain phosphorylation ( SP ), excretes organic fermentation products (OP), which are heterotrophic to H 2 and H 2 and from an acetogenic bacterium ( AB ) CO 2 are fermented. These products are condensed to acetic acid by the Wood-Ljungdahl route ( WL ). Acetogenic bacteria can also absorb and process H 2 and CO 2 from the outside. They compete with obligate autotrophic archaea ( M1 ), which carry out methanogenesis . Acetoclastische, acetic acid "cleaving" methanogens other hand, are in syntrophy to acetogens. They shift the equilibrium 4 H 2 + 2 CO 2 ↔ CH 3 -COOH to the right by removing the end product acetic acid and recycling CO 2 .

Elemental hydrogen (H 2 ) serves as the reducing agent in the acetyl-CoA route . Like CO 2 , it comes from a number of fermentation processes . These either take place in the acetogenic bacteria themselves. They can ferment heterotrophic organic material to H 2 / CO 2 and thus form the reactants for the acetyl-CoA pathway. The entire process of fermentation and acetogenesis is then referred to as homoacetate fermentation . The two gases H 2 and CO 2 are also formed and excreted by other fermenting organisms. They enable acetogenic bacteria to grow in a lithoautotrophic manner .

The exergonic redox reaction 4 H 2 + 2 CO 2 → CH 3 -COOH provides so little energy (ΔG 0 '= -111 kJ / mol) that one can speak of “ autotrophy at the thermodynamic limit”. The ATP necessary for growth is also formed in acetogenesis by an ATP synthase according to the chemiosmotic principle .

Ecological position and occurrence

As anaerobic respiration , acetogenesis only takes place in habitats in which no oxygen (O 2 ) is available for the oxidation of dead organic material. The anaerobic degradation occurs gradually throughout different microorganisms that excrete degradation products, which then other anaerobic serve organisms as a food source. At the end of this food chain are living beings that can still grow even when no more organic material is available. They are able to do this because they can assimilate inorganic CO 2 to build up cell material . They are autotrophic and also lithotrophic , as they use inorganic H 2 to reduce CO 2 .

The CO 2 assimilation is an energetically expensive process. The organisms at the end of the anaerobic food chain are chemotrophic . That means that they don't get energy from light, but from chemical reactions. At the end of the anaerobic respiratory chain, only the two reaction partners that are used for CO 2 assimilation are available to generate energy : CO 2 and H 2 .

These two compounds are the basis of life for two very different groups of organisms: acetogenic bacteria and methanogenic archaea.

Acetogens and methanogens in comparison
feature Acetogens Methanogens
End product acetic acid methane
An energetic response in autotrophic growth 4 H 2 + 2 CO 2 → CH 3 -COOH + 2H 2 O, ΔG 0 'approx. -100 kJ / mol 4 H 2 + CO 2 → CH 4 + 2H 2 O, ΔG 0 'approx. -130 kJ / mol
Alternative substrates Acetogens are by definition autotrophic, some can also use carbon monoxide . As a rule, however, they can also utilize a large number of organic compounds heterotrophically. This happens not only through fermentation, but also through the use of methyl groups in the Wood-Ljungdahl way. Some methanogens can use low molecular weight methyl compounds and carbon monoxide. Acetic acid is the substrate for all acetoclastic methanogens (Methanosarcina), some of which can only grow with this substrate. Formic acid is used by many methanogens.
Electron acceptors of anaerobic respiration CO 2 mostly only serves as an electron acceptor in the absence of stronger oxidizing agents such as nitrate , nitrite , thiosulfate , fumarate , pyruvate and acetaldehyde . Of some (hydrogenogenic) acetogens, even H + can be used as an electron acceptor if z. B. Methanogens keep the concentration of H 2 and CO 2 very low. only CO 2
Occurrence Acetogens occur exclusively in bacteria , mostly Firmicutes of the class Clostridia , but also in Acidobacteria and Spirochaetes in more than 20 genera. Often acetogens are closely related to non-acetogenic bacteria. Methanogens belong exclusively to archaea of the class Euryarchaeota . These are subdivided into Methanobacteriales, Methanococcales, Methanomicrobiales, Methanocellales, Methanosarcinales and Methanopyrales, along with some clearly defined non-methanogenic clusters.

In summary, acetogenic bacteria differ from methanogenic archaea in that the latter, due to their obligatory “CO 2 breathing”, are specialists at the lowest end of the anaerobic food chain. In contrast, acetogenic “CO 2 breathing” is an additional option for organisms that belong to many taxonomic groups. Often it only occurs under certain growth conditions. So it happens that acetogenesis is repeatedly detected in bacteria that were previously considered to be non-acetogenic.

Fig. 2. Clostridium difficile , an actetogenic bacterium.

Acetogenic bacteria are found in many anaerobic biotopes .

Occurrence of some acetogenic bacteria
Biotope organism annotation
ground Clostridium aceticum First isolated acetogenic bacterium (1936)
Feces, rabbits Clostridium autoethanogenum In addition to acetic acid, it also produces ethanol from CO
Intestines, mammals Clostridium difficile Acetogenesis only detected in strains isolated from the rumen of newborn lambs.
Sewage sludge Clostridium glycolicum Acetogenesis demonstrated in some strains. A strain is aerotolerant up to 6% O 2
Feces, birds Clostridium ljungdahlii Can reduce nitrate as an electron acceptor to ammonia.
Sewage sludge Eubacterium limosum When grown from carbon monoxide, it also produces butyric acid
ground Moorella thermoacetica Temperature optimum 55-60 ° C
Volcanic springs Moorella thermoautotrophica thermophilic
Alkaline waters Natroniella Halophilic bacteria that can only use organic compounds to reduce CO 2
Feces, mammals Ruminococcus
Freshwater sediment Holophaga foetida Does not belong to the Firmicutes, but to the Acidobacteria
Termite burrow Treponema primitia. Belongs not to the Firmicutes, but to Spirochaetes
Marine sediment Acetobacterium woodii Nitrogen fixer
Freshwater sediment Sporomusa malonica

Acetogenic Bacteria Biochemistry

Fig. 3. Lithoautotrophic (green arrows) and heterotrophic (blue arrows) metabolism of Acetobacterium woodii.
1 acetyl-coenzyme A, 1a acetyl phosphate, 2 tetrahydrofolate (THF), 2a formyl-THF, 2b methenyl-THF, 2c methylene-THF, 2d methyl-THF, AcKi acetate kinase, CO-Dh CO-dehydrogenase / acetyl-CoA synthase , EBH Elektronenbifurkierende hydrogenase Fdox ferredoxin Fdred ferredoxin 2- , FTCy formyl-THF cyclohydrolase, FTS formyl-THF synthetase Gly glycolysis, HDCR H 2 -dependent CO 2 reductase, MTDe Methylene-THF dehydrogenase, Mtra methyltransferase MTRed Methylene-THF-Reductase, PFdO Pyruvate: Fd-Oxidoreductase, Ptra Phosphotransacetylase , RNF Ferredoxin: NAD-Oxidoreductase (Rnf-Complex)

Many anaerobic organisms fix CO 2 using the acetyl-CoA pathway , in which coenzyme A is converted into “activated acetic acid” ( acetyl-CoA ). During acetogenesis, the endergonically produced acetyl-CoA is not only converted into cellular substance, but rather catabolically with partial energy recovery .

The catabolic processes are shown schematically in Fig. 3. With CO 2 fixation, energy is consumed in two reactions.

1. The reaction of HCOOH with THF ( 2 ) is catalyzed by formyl-THF synthetase ( FTS ) and consumes ATP. In principle, this is regenerated by the acetate kinase ( AcKi ) during the final formation of acetic acid from acetyl phosphate ( 1a ). (Fig. 3, below)

2. When the second CO 2 is fixed , the CO dehydrogenase / acetyl CoA synthase ( CO-Dh ) oxidizes ferredoxin ( Fd ), which is regenerated during heterotrophic growth, for example by pyruvate: Fd oxidoreductase ( PFdO ). During lithoautotrophic growth, however, the ferredoxin endergonic must be regenerated by means of H 2 . The mechanism wasn't cleared up until the second decade of the 21st century:

  • H 2 is fed into the overall process by an electron bifurcating hydrogenase ( EBH ). This cytosolic enzyme regenerates ferredoxin and NADH. H + is released inside the cell, which has to get out of the cell so that it does not become over-acidic. The hydrogenase regenerates NADH from NAD +, synchronously with ferredoxin, according to the principle of electron bifurcation .
  • The NADH is consumed by the methylene-THF dehydrogenase ( MTDe ), as well as by a bifurcating methylene-THF reductase ( MTRed ), which reduces ferredoxin.
  • A membrane-based enzyme, ferredoxin: NAD oxidoreductase ( Rnf complex , RNF ) finally functions as an ion pump according to the chemiosmotic principle . It uses the potential difference between Fd red / Fd ox and NADH / NAD + . The reverse current of the ions drives the regeneration of ATP by a Na + -driven ATP synthase .

Remarks

  1. ↑ Acetogenesis is derived from the Latin acētum = vinegar and ancient Greek γένεσις genesis = origin. The acetic acid mainly formed during acetogenesis is, depending on the pH, in equilibrium with its corresponding base acetate .
  2. ↑ The term carbonate respiration used earlier is misleading because carbonates cannot be reduced directly, but must first be converted to CO 2 . The enzyme carbonic anhydrase, which is also used by acetogenic bacteria, serves to catalyze this reaction. (See also Kerry S. Smith, James G. Ferry: Prokaryotic carbonic anhydrases . In: FEMS Microbiology Reviews . 24, No. 4, 2000, pp. 335-336. Doi : 10.1111 / j.1574-6976.2000.tb00546 . )
  3. ↑ These include u. a. the heterofermentative lactic acid fermentation , the mixed acid fermentation and the propionic acid fermentation .

Individual evidence

  1. ↑ Harold L. Drake, Kirsten Küsel, Carola Matthies: Acetogenic Prokaryotes . In: Eugene Rosenberg et al. (Ed.): The Prokaryotes - Prokaryotic Physiology and Biochemistry . 4th edition. Springer-Verlag, Berlin Heidelberg 2013, p. 4 , doi : 10.1007 / 978-3-642-30141-4_61 .
  2. ↑ a b c Fuchs, Georg: General microbiology . 9th edition. Stuttgart 2014, ISBN 978-3-13-444609-8 , pp. 461-462 .
  3. ↑ Jiyeong Jeong, Johannes Bertsch, Verena Hess, Sunju Choi, In-Geol Choi, In Seop Chang, Volker Müller ,: Energy Conservation Model Based on Genomic and Experimental Analyzes of a Carbon Monoxide-Utilizing, Butyrate-Forming Acetogen, Eubacterium limosum KIST612 . In: Applied and Environmental Microbiology . tape 81 , no. 14 , 2015, p. 4782-4790 , doi : 10.1128 / AEM.00675-15 .
  4. ↑ a b Stephen W. Ragsdale, Elizabeth Pierce: Acetogenesis and the Wood-Ljungdahl pathway of CO 2 fixation. In: Biochimica et Biophysica Acta (BBA) -Proteins and Proteomics . tape 1784 , no. 12 , 2008, p. 1873–1898 , doi : 10.1016 / j.bbapap.2008.08.012 .
  5. ↑ a b Fuchs, Georg: General microbiology . 9th edition. Stuttgart 2014, ISBN 978-3-13-444609-8 , pp. 435-438 .
  6. ^ Edward Schwartz, Johannes Fritsch, Bärbel Friedrich: H 2 -Metabolizing Prokaryotes . In: Eugene Rosenberg et al. (Ed.): The Prokaryotes - Prokaryotic Physiology and Biochemistry . 4th edition. Springer-Verlag, Berlin Heidelberg 2013, p. 119-200 , doi : 10.1007 / 978-3-642-30141-4_61 .
  7. ↑ Debabrata Das & T. Nejat Veziroǧlu: Hydrogen production by biological processes: a survey of literature . In: International Journal of Hydrogen Energy . tape 26 , no. 1 , 2001, p. 13-28 , doi : 10.1016 / S0360-3199 (00) 00058-6 .
  8. ↑ Kai Schuchmann, Volker Müller: Autotrophy at the thermodynamic limit of life: a model for energy conservation in acetogenic bacteria . In: Nature Reviews Microbiology . No. 12 , 2014, p. 809-821 , doi : 10.1038 / nrmicro3365 .
  9. ^ Eva Biegel: Conservation of energy in Acetobacterium . In: BIOspectrum . No. 4.12 , 2012, p. 453 , doi : 10.1007 / s12268-012-0209-5 .
  10. ↑ Harold L. Drake, Kirsten Küsel, Carola Matthies: Acetogenic Prokaryotes . In: Eugene Rosenberg et al. (Ed.): The Prokaryotes - Prokaryotic Physiology and Biochemistry . 4th edition. Springer-Verlag, Berlin Heidelberg 2013, p. 3-61 , doi : 10.1007 / 978-3-642-30141-4_61 .
  11. ↑ Harold L. Drake, Kirsten Küsel, Carola Matthies: Acetogenic Prokaryotes . In: Martin Dworkin, Stanley Falkow, Eugene Rosenberg, Karl-Heinz Schleifer, Erko Stackebrandt (Eds.): The Prokaryotes - Prokaryotic Physiology and Biochemistry . 3. Edition. Springer-Verlag, New York, NY 2006, ISBN 978-0-387-30742-8 , pp. 354-420 , doi : 10.1007 / 0-387-30742-7 .
  12. ↑ Harold L. Drake, Kirsten Küsel, Carola Matthies: Acetogenic Prokaryotes . In: Eugene Rosenberg et al. (Ed.): The Prokaryotes - Prokaryotic Physiology and Biochemistry . 4th edition. Springer-Verlag, Berlin Heidelberg 2013, p. 9-45 , doi : 10.1007 / 978-3-642-30141-4_61 .
  13. ↑ Kirsten Küsel, Tanja Dorsch, Georg Acker, Erko Stackebrandt, Harold L. Drake: Clostridium scatologenes strain SL1 isolated as an acetogenic bacterium from acidic sediments. In: International Journal of Systematic and Evolutionary Microbiology . 50, pp = 537-546, 2000.

See also